people being supervised often do things they are not supposed to do when the supervising person is not around (expression often used in a humorous way about employer and employees, parents and children, etc.)
WHEN THE CAT'S AWAY THE MICE WILL PLAY
Meaning of WHEN THE CAT'S AWAY THE MICE WILL PLAY in English
Explanatory English dictionary bed edition. Толковый словарь английского языка Редакция bed. 2012